The invention relates to a novel type of molding element intended to be fitted in a mold for molding a cutout in a tread of rubber mix intended to be fitted on a tire, the invention also relates to a mold equipped with this type of molding element.
French Patent Application No. 98-08653 discloses a molding element for molding in a tread an incision of cylindrical geometry defining a rubber block, the lateral wall of which is connected to the rest of the tread by rubber bridges.
In one variant presented in the aforementioned application, a molding element comprises at least two molding parts of generally cylindrical shapes said molding parts being mounted freely in rotation relative to each other about one and the same axis XX'. A first cylinder bears at least one molding blade wound at a first angle around the axis XX' to form a helix and a second cylinder concentric to the first comprises at least one other molding blade wound around the same axis XX' at a second angle of a sign opposite to the sign of the first angle. In this latter configuration, it is envisaged that the molding blades of said two cylinders are arranged such that the blades of the first cylinder and those of the second cylinder are in sliding contact with each other so as to form a sort of trellis defining a plurality of orifices intended to allow the rubber mix to pass during molding to form connecting bridges between the lateral wall of the molded block and the tread.
In this molding element proposed in the aforementioned application, the demolding of a tread after molding and vulcanization, that is to say, the extraction of said molding element from said tread, is made possible without breaking the bridges molded by the structure of the molding parts and their free mounting in rotation relative to the mold. In actual fact, the forces exerted by the rubber on the helical molding blades, during the demolding, create a resultant torque which causes the molding parts to turn about their axis XX'.
With this latter arrangement, it was noted, in particular after several molding operations, that the contact between the blades was insufficient and that films of rubber mix formed between said blades during molding. Consequently, after molding not a single incision but at least two non-interconnected incisions are obtained. The films of rubber mix form walls between the incisions which prevent the free circulation of fluids (air, water) from one incision to the other and isolate them from each other.
Furthermore, the forces exerted by the raw rubber mix at the time of molding and of penetration of the blades into said mix result in deformation of the molding blades, the consequence of which is an increase in the thicknesses of the molded films, and in gradual modification of the geometry of said blades. This deformation is all the more marked since the blades used are of low thickness compared with their length (which is frequently the case for molding incisions of a width of less than 2 mm).
The same defects are noted in the case of molding an incision which does not completely define a rubber block but has a trace on the outer surface of the tread, the geometry of which is in the form of a circle.